Concrete embedded retaining walls such as diaphragm walls, known as slurry walls in the USA, have been part of foundation construction for sixty years. Forming the joint between successive panels has always been one of the most difficult and time consuming elements of the process. Existing construction methods of forming joints involves using and then removing stop-ends. Pre cast stop-ends have been used occasionally.
In the early days of diaphragm wall construction the individual panels were dug with grabs with rounded clams so steel pipes, the same diameter as the thickness of the wall, were placed at the ends of the panels and extracted after concreting leaving a round hole filled with slurry. The hole helped guide the grab digging the adjacent panel and the system provided a semi-circular concrete construction joint between adjacent panels.
As diaphragm walls became thicker and deeper so the steel pipes became bigger, longer and heavier requiring jointing systems to connect individual sections and jacking equipment to extract the pipe from the ground. As the depth of diaphragm walls increased, so the timing of this extraction process became more critical. Too soon and the unset concrete collapsed into the void, too late and the pipe became stuck fast into the hardening concrete. Great skill and experience was required to manage the process and diaphragm wall projects routinely worked late into the night.
As the use of diaphragm walls became more widespread, alternative shapes of joint formers came into use. The round ended digging grabs gave way to the more efficient square ended variety. Companies started using a joint former which was shaped like a rectangle with an equilateral triangle on the concrete face. On occasion “Organ Pipe” joint formers were used. Both of these shapes were easier to extract than the earlier circular formers.
Towards the end of the 1980s and into the early 1990s two developments changed how diaphragm wall panel joints were formed. One of the developments was the “Hydrofraise” now more commonly known as a “hydro-mill” or cutter. The cutting/milling wheels on this machine can cut into concrete if there is equal resistance to the wheels on both sides of the machine during the progress of the excavation. If the machine is cutting equally into the concrete at their ends of two already constructed panels then a straight construction joint between the newly excavated panel and the already concreted panels on each side of it can be formed. The degree of panel to panel contact is determined by the excavation verticality that can be achieved. This joint system is now predominantly used in deep circular shafts where the walls are working in hoop stress so the joints are in compression making water leaks less likely and making shear keys unnecessary.
Two other examples of embedded concrete retaining walls are secant pile walls and contiguous pile walls. Secant pile walls have a row of bored piles, primary piles, installed with spaces between each pile. Another row of bored piles, secondary piles, is inserted into the spaces between the first row of piles however the spaces are smaller than the diameter of the secondary piles so a cut is made into the concrete of the piles on either side thus forming a continuous wall. A contiguous pile wall is a single row of piles with a small (usually less than 500 mm) space between them and is used as a retaining wall system where it is not necessary to hold back groundwater. These wall systems are generally used to depths of about 25 m because deviation from the vertical during installation can result in gaps between secant piles and unacceptable large spaces between contiguous piles. Secant pile walls are sometimes used to form circular shafts but as a minimum width concrete to concrete contact, between primary and secondary piles, is required to develop the hoop stress required by the design then any vertical deviation of individual piles is likely to become unacceptable at relatively shallow depths. For this reason secant pile shafts are usually no more than 10 m to 15 m deep. Secant pile walls, for example to form a shaft, typically have a row of bored piles installed with spaces between each pile. Another row of bored piles is inserted into the spaces between the first row of piles however the spaces are smaller than the diameter of the piles so a cut is made into the concrete of the piles on either side thus forming a continuous wall. A contiguous pile wall is just a row of piles with a small (usually less than 500 mm) space between them and is used as a retaining wall system where there is no problem with ground water. Accurate positioning of the piles in these systems can be time consuming and difficult to achieve.
Hydro-mills have great difficulty cutting into concrete at only one end of a trench. The differing resistance to excavation progress at one end of the trench, compared to the other end, is difficult to manage and leads to unacceptable deviations in the verticality of the excavation.
The other development was the system known as CWS or continuous water stop joints. In this system an end former, being steel plate with a steel trapezoid shape on one face, is supported from the guide wall with the flat side against the soil at the ends of the panel excavation. In the middle of the trapezoid a fabricated clamp arrangement holds a rubber water bar half of which is concealed in the end former. The protruding half of the water bar is then cast into the concrete. The joint former is later peeled away from the concrete during and after excavation of the adjacent panel leaving a shear key formed by the trapezoid shape with half the dumbbell water bar protruding out ready to be cast into the concrete of this next panel. This system had several major advantages over the earlier extraction systems:                No late working and overtime to extract the joint        Better water tightness because of the rubber water bar that can now be introduced into the joint.        Complete panel to panel connection. Not always the case with the extraction systems where grab operator experience and competence was also a major factor.        Simpler to use and less risk therefore requiring less experienced and less skilled personnel.        
The system was initially tried on relatively shallow 20 m to 25 m deep diaphragm walls. By the mid 90's wall depths of 30 m to 40 m were using the system although now problems started to arise. Sometimes the former was difficult to peel off taking hours and in some cases days. A few formers broke with portions left behind in the joint. It became clear that if the former was slightly buckled or distorted in any way or if it was not correctly positioned and suspended or if the excavation was out of position/verticality then the wedges, grabs and/or chisels used to remove it would become less efficient (e.g. due to jamming in the excavated panel).
The problems worsened with the recent switch from rope grabs to hydraulic grabs that has occurred over the last twenty years. While there is no doubt that the modern steerable hydraulic grab digs faster and more accurately than the old rope grabs, the weight, lack of free fall capability, hydraulic connections etc. do not allow the equipment to be used as an effective chiselling tool which is really what is required to peel off a CWS former.
With special precautions the system has been used to depths in excess of 50 m but the skill and experience required to do this is not easily found and even if possessed cannot always be present. Delays in or failure to remove the former can have major cost and programme implications for a project.
For the reasons stated in the previous paragraph on projects where the diaphragm wall has been over 35 m deep, and the panels have either been excavated with grabs or with hydro-mills but without overcut joints, precast concrete “stop-ends” have been used. This does reduce risk but at some cost penalty partly from the manufacture and transport of the precast concrete sections and partly because their weight may require additional or larger cranes on site to lift and place the units. In one sense precast concrete “stop ends” are a retrograde step. This is because double the number of joints in any wall increases the risk of leakage and the nature of the stop end construction does not lend itself to effective incorporation of water bars further compromising water tightness. There is also greater potential for misaligned panel connections because of the difficulty of incorporating an effective grab guide in the relatively thin precast concrete section. Despite the obvious disadvantages of using precast concrete stop ends, companies have opted for their use on recent projects because of the risk associated with the use of the CWS system at depths over 35 m to 40 m.
Modern hydraulic diaphragm wall grabs are capable of digging to depths of over 60 m with a high degree of positional accuracy. Diaphragm wall panel jointing systems have not kept pace with the development of the grabs. As depths increase above 30 m so reluctance by contractors to use the CWS system increases. The alternative of using precast concrete stop ends is costly and technically inferior.
Prior art excavator grabs or mills are used to excavate the trench and will typically exert a digging or cutting force (and therefore encounter balancing resistance) on the digging teeth at both ends at once of the grab bucket halves, or on the cutting teeth on the surface of the two opposing cutting wheels of the hydro-mill (typically exerting equal cutting force on both sides of the grab bucket halves, or opposing cutting wheels). Thus, as the excavation proceeds, the excavating grab or mill does not veer off to or away from a cutting face due to less or more resistance being encountered on the other side of it. Unless the grab or mill is excavating the same material at each side, the excavating grab or mill will veer off away from the harder cutting side due to less resistance being encountered on the other.
FR2594864 ROCHMANN describes a method of casting a wall in the ground using a profile.
U.S. Pat. No. 4,582,453 RESSI describes in situ forming of underground panel walls with improved joint structure.U.S. Pat. No. 4,930,940 and EP0333577 SONDAGES describe a guiding system for constructing a wall cast in the ground. Wheels are used to clear concrete from the guide member.
EP0101350 SONDAGES describes a procedure and mechanism for withdrawal of a shuttering mechanism used to prepare an end face of concrete panel.
EP0649716A CASAGRANDE describes a cutter for forming diaphragm joints having a cutting assembly and a thrust and guide assembly.
U.S. Pat. No. 4,838,980, DE3430789, U.S. Pat. No. 4,990,210 GLASER describes a method and apparatus for introducing and joining diaphragms in slotted walls in which the interior of connecting pipes are rinsed free of support fluid. DE 3503542 GLASER describes a link for panels.
GB2325262 KVAERNER describes a hydrophilic waterbar for diaphragm wall joints.
EP0411682 VERSTRATEN describes a retention wall and procedure for making a liquid tight wall in the ground.
EP0580926 MIATELLO RODIO describes a sealing joint in a diaphragm formed by concrete panels. An inner core is extracted from a joint member following removal of a guide tube end stop.
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US2002/0119013 SHOTTON describes a waterstop for foundation elements.
CN101560767 LIXIN TAN describes a method of connecting slotted sections.
GB1590325 COMAR REG TRUST describes a metal shuttering member in the form of a prism of generally rectangular section.
FR2708946 SPIE describes a watertight joint between two panels.
U.S. Pat. No. 4,367,057 HUGHES describes drilling a bore between adjacent-sections.
CN101858090 describes soft connection of diaphragm wall joints using rigid joint flexible filler.
The prior art above does not address many of the problems outlined above. The present invention seeks to alleviate one or more of the above problems